Presently, modems are used to transport digital data between data terminal equipment such as personal computers, workstations and the like, over channels, such as telephone lines. Typically, the communication signal includes digital data in the form of a carrier signal that is modulated by a predetermined transmit constellation of signal points. Each signal point of the transmit constellation represents a digital code or value to be transported. At the sending modem, a carrier signal is thus modulated at the constellation signal point corresponding to the digital code or value to be transported over the channel. At the receiving modem, the modulated carrier signal may include distortion and other impairments induced as a result of the transmission process. The carrier signal is removed, and a received signal point is then identified from a receive constellation. The digital code or value represented by the signal point may then be obtained from a memory device, such as a look-up table.
In the past, typically such modems have been used with voice-grade analog telephone lines. However, modern telephone networks are increasingly migrating from analog systems to digital systems. For instance, it is common for large telephone users to be served by a T1-carder digital service. In fact, most telephone companies today not only offer such service, but encourage their larger customers to order this service.
As is known, present T1 systems are capable of providing a group of 24 digital channels to an end user, each channel consisting of a stream of 8-bit samples, each sample transported at 8,000 samples/second, in a telephone company-specified pulse code modulation ("PCM") format defined as "DS-0". This is roughly equivalent to 64,000 bits/second ("64 Kbps") per channel. Thus, it is possible for a customer to order a single T1 connection from, for instance, their premises to a telephone company-owned network switch for distribution of the 24 channels of 64 Kbps to other locations. This would allow a customer to connect to a telephone network switch with one T1-carrier and send calls to 24 different locations.
In this arrangement, each of the 24 channels could be digital data or voiceband analog signals in the DS-0 format. This method of access allows the customer to send the same amount of data over one pair of wires, namely, the T1 service, instead of 24 separate pairs. This provides a savings to the network provider and also to the customer. It also presents opportunities for innovations in equipment design, since a modem and a channel bank card can be replaced by a single card including both a modem and modem channel bank converter.
For a variety of reasons, however, the customer may still need to terminate at least one end of a DS-0 channel with a modem. One problem with this is, of course, that the modem typically operates at 9,600 samples per second, whereas each DS-0-formatted channel of the T1 channel bank operates at 8,000 samples per second.
Prior art arrangements have been devised to convert the 9,600 Hz modem samples to 8,000 Hz channel bank samples. One such prior art arrangement utilizes series-connected converter units. In this arrangement, the modem 9,600 Hz digital samples are first processed by a digital-to-analog (hereinafter "D/A") converter to convert the modem samples to analog form. Once converted to analog form, the modem signal is then processed by an analog-to-digital (hereinafter "A/D") converter to re-convert the modem signal back to a digital form, comprising 8,000 Hz samples. Once convened to 8,000 Hz digital samples, this resulting signal is finally applied to the channel bank.
The problem with this prior art arrangement is the signal-to-noise level of the information signal is impaired by the series-connected converter units. Such an impairment can be significant, in view of the fact that a typical converter, with its required analog circuitry, adds approximately 1.0 dB of noise to the processed signal.
As a result, there is a need for a converter to directly convert modem samples to T1 channel bank samples, where the modem and the channel bank are operating at different frequencies.